1. Field of the Invention
The present invention relates to optical disks, and more particularly to bonded-type optical disks excellent in the life quality.
2. Prior Art
In general, an optical disk comprising two substrates, each having a recording layer or a reflective layer on its surface, is advantageous in that a recording capacity is large and is widely used as a video disk, a digital audio disk, or a document or data file disk. FIG. 9 is a perspective cross-sectional view schematically showing a conventional bonded-type optical disk. FIGS. 10A and 10B show schematic cross sections of the bonded disks constituting a bonded-type optical disk. More specifically, a conventional bonded-type optical disk 30 comprises a light-permeable substrate 9 having a thickness of about 1.2 mm. Signal pits or tracking grooves are formed on one surface of the substrate 9. A recording layer 10 is provided on the signal-pit or tracking-groove formed surface of the substrate 9. A protection film 11 is provided on the recording layer 10 if necessary. FIG. 10A shows a double-sided optical disk for use in both of the recording and the reproduction. Two substrates 9, each being provided with the recording layer 10, are bonded at their opposing faces via an adhesive layer 12. Their recording layers 10 are interposed between the opposing substrates 9. FIG. 10B shows a single-sided optical disk for use in both of the recording and the reproduction characterized by the structural features that only one of two substrates 9 is provided with the recording layer 10.
The substrate 9 made of a light-permeable material, such as an optical glass, an acrylic resin, a polycarbonate resin, and an amorphous polyolefine resin. The recording layer 10 is made of a photoelectromagnetic recording material, such as TbFe, GdCo, GdTbFe and MnCuBi, or a phase-change recording material, such as As--Te--Ge group and Sn--Te--Se group, which are basically or rewritable materials, or made of a write-once type material represented by a cyanine group or a phthalocyanine group material, otherwise made of an Al-group, Cu-group or Ag-group reflective layer for a reproduction-only type disk on a substrate of which signal pits are formed beforehand. The protection film 11 is chiefly made of a ultraviolet hardening resin. The adhesive layer 12 is chiefly made of a hotmelt-type adhesive and a ultraviolet hardening type adhesive (referred to UV adhesive, hereinafter).
A problem always occurring in handling optical disks is a deviation of the disk. The above-described bonded-type optical disks are not exception in that they are subjected to this problem too. The conventional bonded-type optical disk, as described above, has the structural features that two substrates, each having a thickness of about 1.2 mm, are bonded directly or via a spacer. In addition, a recent trend toward high densification forces the optical disks to reduce the thickness of their substrates. Reduction in the thickness of the substrate leads to the reduction of the rigidity of the optical disk. This is disadvantageous in eliminating the deviation. Especially, for a one-sided recording/reproduction type optical disk (shown in FIG. 10B), there is a tendency that a large deviation is produced due to its structural unbalance or asymmetry. As another factor causing such a deviation is the characteristic parameters of an adhesive used for bonding the substrates.
For example, an adhesive used in a conventional bonded-type optical disk is the type PS450-20 commercially available from ACI JAPAN LTD. A sample of the one-sided recording/reproduction type optical disk using PS450-20 is experimentally fabricated by bonding 0.6 mm thick polycarbonate substrates by this adhesive PS450-20. The fabricated optical disk is then subjected to an environmental test under the condition of the temperature 55.degree. C., humidity 95% and 96 hours. After finishing the environmental test, a time-sequential change in the deviation of the tested disk is measured. FIG. 8 shows the result of the measurement. As apparent from FIG. 8, the deviation of the tested disk varied about 1.0 degree before and after the environmental test.
Other hotmelt-type adhesive conventionally used is, for example, the type PS450-56 commercially available from ACI JAPAN LTD., the type PPET-2101 commercially available from TOAGOSEI CO., LTD., or other EVA group or olefine-group adhesives. The similar environmental tests were conducted on these adhesives, and it was found that a large deviation was caused in the same manner.
Other conventional adhesives, used as UV adhesives, are ARONIX UV-3610, 3810, M-210, M-240, M-1100, M-1200, M-1400, M-6100, M-6200, M-6250, and M-6300 commercially available from TOAGOSEI CO.LTD., cyclohexyl (metha) acrylate, poly(ethylene glycol).about.(metha) acrylate, 1.6-hexanediol.about.di (metha) acrylate, neopentyl glycol.about.di (metha) acrylate, hydroxy pivallic acid neopentyl glycol.about.di (metha) acrylate, EO modified bisphenol A diacrylate, and isobomyl (metha) acrylate. These adhesive materials are similar to the hotmelt-type adhesives in that a large deviation is caused.
As described above, conventional bonded-type optical disks cause a large change in the angular deviation when the operating environments are suddenly changed. In accordance with the requirement of high densification, a margin of the player to the deviation of a disk is becoming small. Furthermore, there is a tendency that the optical disks are preferably adopted to the peripheral devices of a computer. Thus, the environmental durability of the optical disk needs to be further improved to endure severe operating conditions.